The present invention relates to a method and apparatus for combining measurements from different sensors in order to provide an improved measurement of a parameter. It is particularly applicable to the measurement of physiological parameters.
Certain parameters can be measured in more than one way. This is useful in giving independent measures of the same quantity. For instance, in the medical field the heart rate can be measured both from an electrocardiogram (ECG) and from a pulse oximetry waveform (used to calculate oxygen saturation). drawings FIG. 4 illustrates schematically these-two-waveforms, FIG. 4a being the electrocardiogram with the heart rate illustrated as HR1, and FIG. 4b the pulse oximetry waveform with the heart rate illustrated as HR2. The heart rate is a parameter which can undergo sudden changes. Some of these changes are valid physiological changes, for example ectopic beats which occur prematurely, and therefore give rise to a temporary increase in the heart rate. FIG. 5 illustrates the occurrence of an ectopic beat 50 found in both the electrocardiogram trace and the pulse oximetry waveform. The shorter interval between the preceeding beat and the ectopic beat 50 manifests itself in a measurement of the heart rate as a sudden increase in the heart rate. FIGS. 1 and 2 of the accompanying drawings show time plots of the heart rate measured by pulse oximetry (FIG. 1) and ECG (FIG. 2). It can be seen that in FIGS. 1 and 2 the heart rate in. the early part of the plot is generally of the order of 80 bpm, but that there are occasional sudden increases in heart rate, such as indicated at 10 and 20 which are caused by ectopic beats and thus appear both in the measurement by pulse oximetry and the measurement by ECG.
However, in addition to changes in the measured heart rate deriving from valid physiological changes, other changes occur which are not physiologically valid, for instance being caused by sudden movement of the sensors on the body surface (e.g. chest movement with ECG electrodes). FIG. 6 illustrates the presence of artefacts 60 on the pulse oximetry waveform which shorten the interval between apparent beats and thus result in apparent increases in the heart rate. These changes are reflected in one measurement, but not the other, as indicated at 12 and 22 in FIGS. 1 and 2 respectively. The fact that the changes appear in one measurement but not the other means that the two measurements could be combined to help decide which heart rate changes are valid physiological ones, and which are artefacts. However, the normal approach of validating one measurement channel against the other involving cross-correlation of the two measurements invariably fails because it is not possible to know in advance (for each recording, for each patient) what value to give to the threshold for accepting, rather than rejecting a change in the heart rate as being valid. Thus although it would appear from FIGS. 1 and 2 that a threshold could be set which would eliminate changes such as indicated as 22, such a threshold is not appropriate for all patients for all recordings, and does not help with the pulse oximetry waveform. The problems are increased in the event of atrial fibrillation when the heart rate changes rapidly as indicated in the region AF in FIGS. 1, 2 and 3.
Similar problems arise in other fields where a parameter is measured via two or more measurement channels.
The present invention provides a method and apparatus for improving measurement of a parameter by combining two measurements of it in a way which allows valid changes to be distinguished from artefacts. Accordingly it provides a method of measuring a parameter comprising the steps of: predicting the value of each of two measurements of the parameter, making two measurements of the parameter to produce two measured values of the parameter, calculating the respective differences between the predicted values and the measured values, and combining the two measured values with weights determined by said differences.
Thus with the present invention a prediction is made for each measurement and the actual measurement is compared with its prediction. The difference is computed, which is termed the “innovation”, and this innovation is used to calculate a weight which will be given to that measurement when it is combined with the other measurement, also weighted according to its innovation. The weights are calculated so that if the innovation on one measurement channel is high, whereas the innovation on the other measurement channel is low, the measurement from the low innovation channel is more heavily weighted. This is because a high level of innovation from one channel coinciding with a low innovation on the other channel is regarded as indicative of an artefact on the higher innovation channel. Thus, the weight given to each value when the values are combined is inversely related to the square or modulus of the difference between the measured value and its predicted value.
In one embodiment the measured values can be combined according to the formula:
                    M        =                                            M              1                        ⁢                                          σ                2                2                                                              σ                  1                  2                                +                                  σ                  2                  2                                                              +                                    M              2                        ⁢                                          σ                1                2                                                              σ                  1                  2                                +                                  σ                  2                  2                                                                                        (        1        )            where M1 and M2 are the two measured values, and σ1 and σ2 are the differences between the two measured values and their respective predicted values.
The steps of prediction, measurement, calculation and combination are preferably repeated continuously, with the predicted value for each of the measurements being based on a linear predictive model, e.g the predicted value is based on its preceding predicted value and the preceding innovation (i.e. the difference between the preceding predicted value and the preceding measurement). The predicted value can be obtained by adding to the preceding predicted value a constant times the innovation. The constant is preferably a positive value less than or equal to unity. Alternatively the predicted value for each of the two independent measurements can be calculated by using. a non-linear, predictive model such as a neural network.
In one embodiment the predicted values can be based on a mathematical model of the system, which may include estimates for process noise and sensor (measurement) noise. Two independent models may be used, one for each of the measurement channels, and the models can include estimates for the process noise and sensor noise, which can be the same for the two channels. In one embodiment the models are Kalman filters.
The method is particularly applicable to the measurement of heart rate, in which case the two measurement channels can be from an electrocardiogram and a pulse oximetry waveform, though it is applicable to any other measurement of a parameter which can be derived from two or more sources. Thus the method is applicable for more than two measurement channels, and both where the measurements are independent and where they are not truly independent such as from multiple leads of an ECG.
The invention can also provide for detection of movement artefacts. In this instance high values of innovation are obtained on both channels for the period of movement, and this can be used as a trigger to discard the sections of data which are corrupted by that movement.
It will be appreciated that the invention can be embodied using computer software and thus the invention extends to a computer program for controlling and executing the method or parts of it, and to a computer readable storage medium carrying the program. The invention also extends to corresponding apparatus for carrying out the method.